Power split transmission with energy recovery

ABSTRACT

Power split drive (PSD) transmissions capable of energy recovery and suitable for use in automotive applications. Each PSD transmission includes a mechanical transmission system for mechanically transmitting mechanical power between a rotatable input shaft and a rotatable output shaft, and a hydraulic transmission system containing a fluid for hydraulically transmitting hydraulic power between the input shaft and the output shaft, and at least a third shaft operatively interconnected to one of the mechanical and hydraulic transmission systems. The hydraulic transmission system is operatively coupled by at least a first planetary gear train to the mechanical transmission system. According to the invention, the PSD further comprises means operatively associated with at least one of the mechanical and hydraulic transmission systems for storing and releasing energy within the PSD transmission, the energy storing and releasing means comprising a flywheel or an accumulator or a combination thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/825,336, filed Sep. 12, 2006, and U.S. Provisional Application No.60/890,536, filed Feb. 19, 2007, the contents of both are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to power transmission systems.More particularly, this invention relates to power split transmissionsystems adapted for automotive applications, in which an energy recoverycapability is provided by the inclusion of one or more energy storagedevices.

Power transmission systems typically found in common automotiveapplications utilize a mechanical transmission system entirely made upof solid components including shafts, gears, and clutches, which alonemay be used to transmit power to the drive wheels of a vehicle, as inthe case of “manual” transmissions. Mechanical transmission systems arealso used in combination with hydraulic transmission systems that use aliquid under pressure to transmit power, as in the case of an“automatic” transmission that uses a torque converter as a hydrodynamicfluid coupling.

Developments in the automotive industry, including passenger,commercial, and off-road vehicles, are shaped by a strong demand toreduce fuel consumption. In addition, there is a trend toward higher topspeed capabilities in heavy off-road vehicles, where faster drivingspeeds on roads are desired. In response, different types ofcontinuously variable transmissions (CVT) have been developed andbrought to the market. Among the CVT concepts, hybrid drives are ofparticular interest to vehicle manufactures. Hybrid drives are based onthe utilization of brake energy for vehicle propulsion by storing brakeenergy in a battery, fly wheel, hydraulic accumulator, or other energystorage means. By recapturing energy otherwise lost as heat duringbraking, the hybrid drive technology is capable of significantlyreducing fuel consumption, particularly in heavy trucks and cars.Comprehensive overviews of hydraulic hybrid drives are provided inStecki et al., “Advances in Automotive Hydraulic Hybrid Drives,”Proceedings of the Sixth International Conference on Fluid PowerTransmission and Control, Hangzhou, China (2005), and Miller,“Comparative Assessment of Hybrid Vehicle Power Split Transmissions,”4th VI Winter Workshop Series (2005). While CVT's provide seamlessshifting in vehicle operation, allowing the engine to operate at anominal speed range resulting in lower fuel consumption and emissions,typical CVT's suffer from either low shaft-to-shaft efficiency or lowtorque handling capabilities.

Power split transmissions (PST), also known as power split drive (PSD)transmissions, are a particular type of CVT that has found use inapplications such as agricultural tractors, for example, the Fendt Varioline of tractors brought to the market by Fendt (AGCO Corporation) in1996. See, for example, Dziuba et al., “Entwicklung eines neuenstufenlosen Schleppergetriebes mit hydrostatisch mechanischerLeistungsverzweigung,” VDI-Berichte Nr. 1393, VDI-Verlag, Düsseldorf,Germany (1998), p. 541-549 (in German). Although the principle of powersplit drives has been known for more than four decades, this technologyis still in a developmental stage.

As known, PSD transmissions traditionally use a planetary (epicyclic)gear train (PGT) in combination with a continuously variabletransmission that achieves continuous variable speed control along withhigh efficiency levels that are derived from the mechanical gears of thePGT. There are three basic implementations of power split drives: thecombination of a PGT with a continuous variable mechanical gear; thecombination of a PGT with a hydrodynamic transmission; and thecombination of a PGT with a hydrostatic transmission. The last of theseallows further fuel savings if a drive line control concept isimplemented that takes engine characteristics into account. The enginespeed can be adjusted to a point where the total power loss of thetransmission is minimized, as reported in Ossyra et al., “Drive LineControl for Off-Road Vehicles Helps to Save Fuel,” SAE InternationalCommercial Vehicle Engineering Congress, Chicago, Ill., USA, SAETechnical Paper 2004-01-2673 (2004). Through the improvement of theefficiency of positive displacement machines, the use of hydrostatictransmissions in PSD's has become very attractive for many differentapplications.

PSD transmissions have three different operating modes that are known inthe automotive transmission industry under the following designations:power additive, full mechanical, and power recirculation. The powerflows of these three modes are schematically represented in FIG. 1. Inthe power additive mode, the power, P_(in), from a combustion engine (orother suitable power source) is split and transferred into two paths: amechanical path (containing a planetary gear train), P_(mech), and ahydraulic path (such as a hydrostatic transmission), P_(hyd). The poweris then combined and transferred as P_(out) to the wheels to propel avehicle. In full mechanical mode, power is entirely transferred from theengine to the wheels via the mechanical path (P_(mech)) and not throughthe hydraulic path (P_(hyd)). Generally full mechanical mode is at asingle speed or has a small speed range. In the power recirculationmode, some of the power transferred via the mechanical path (P_(mech))is recirculated back through the hydraulic path (P_(hyd)). Therecirculated hydraulic power (P_(hyd)) is combined with the engine power(P_(in)) from the engine and again transferred via the mechanical path(P_(mech)), thus being recirculated. In general, full mechanical mode isconsidered to be the most efficient transmission power mode for a PSDtransmission, and the power recirculation mode is considered to be theleast efficient transmission power mode because large amounts of powercan be recirculated through the hydraulic path.

FIG. 2 identifies PSD transmissions categorized by families based onstructural similarities—first, whether the hydraulic path (P_(hyd)) iscoupled to the input (input coupled) or to the output (output coupled)of the mechanical transmission system, and then further categorized bythe characteristics of the planetary gear train (basic, multistage, orcompound). A comparison of achievable efficiencies and operatingcharacteristics of these structural approaches has been presented inCarl et al., “Comparison of Operational Characteristics in Power SplitContinuously Variable Transmissions,” SAE Commercial Vehicle EngineeringCongress and Exhibition, Chicago, USA, SAE Technical Paper 2006-01-3468(2006).

The basic output and input-coupled types are represented in FIGS. 3 and8, respectively. FIG. 3 shows a basic output-coupled PSD transmission 10as utilizing a hydrostatic transmission 12 as the hydraulic path andcontinuously variable part of the transmission 10. The hydrostatictransmission 12 is mechanically coupled (via a gear set) to the outputof a simple planetary gear train 14 (i.e., not multistage or compound),which serves as the mechanical transmission system (path) of thetransmission 10. The planetary gear train 14 is mechanically coupled(via a shaft) to a combustion engine 11 as the power source of thevehicle in which the transmission 10 is installed. The outputs of thehydrostatic transmission 12 and planetary gear train 14 are bothmechanically coupled (via a gear set and a shaft, respectively) to thedrive axle and wheels 19 of the vehicle. The hydrostatic transmission 12comprises two positive displacement units 16 and 18, labeled “Unit 1”and “Unit 2” in FIG. 3. As understood in the art, the positivedisplacement units 16 and 18 operate by trapping and then displacing afixed volume of hydraulic fluid. As such, the speed of the vehicle canbe controlled by controlling the displacements of the units 16 and 18using the vehicle velocity as a feedback signal. The output-coupledtransmission 10 of FIG. 3 is limited to two operational modes: poweradditive and power recirculation. The output-coupled transmission 10operates in the power additive mode at low speeds and the powerrecirculation mode at high speeds. With constant engine speed,increasing the vehicle forward velocity from standstill is achieved byincreasing the displacement of the unit 16 from zero to maximum, thendecreasing the displacement of the unit 18 from maximum to zero. Reverseis achieved by running the unit 16 over center. The differentialpressure in the hydrostatic transmission 12 is determined by the loadtorque. During braking, the high pressure and low pressure lines switchas the units 16 and 18 change pumping and motoring modes.

In FIG. 8, a basic input-coupled PSD transmission 50 is shown asutilizing a hydrostatic transmission 52 as the hydraulic path andvariable part of the transmission 50, and a planetary gear train 54 thatserves as the mechanical path of the transmission 50. In contrast to theoutput-coupled PSD transmission 10 of FIG. 3, the hydrostatictransmission 52 and the planetary gear train 54 are both mechanicallycoupled (via a gear set and a shaft, respectively) to a combustionengine 51, the hydrostatic transmission 52 is mechanically coupled (viaa gear set) to the input of the planetary gear train 54, and the outputof only the planetary gear train 54 is mechanically coupled (via ashaft) to the drive axle and wheels 59 of the vehicle. Similar to thetransmission 10 of FIG. 3, the speed of the vehicle is controlled bycontrolling the displacements of two positive displacement units 56 and58 of the hydrostatic transmission 52, labeled “Unit I” and “Unit II” inFIG. 8, using the vehicle speed as feedback.

As with the output-coupled transmission 10 of FIG. 3, the input-coupledtransmission 50 is limited to two operational modes: power additive andpower recirculation. The transmission 50 operates in power recirculationmode at low speeds, and at higher speeds operates in power additivemode. This cycle can be repeated several times by adding clutches andmore advanced planetary gear trains (e.g., multistage and compound).With constant engine speed, increasing the vehicle forward velocity fromstandstill is achieved by increasing the displacement of the unit 56from negative maximum through zero to positive maximum, then decreasingthe displacement of the unit 58. Reverse is achieved by holding thedisplacement of the unit 56 at maximum and decreasing the displacementof the unit 58. The differential pressure within the hydrostatictransmission system 52 is simply a reactionary item, a function of theload torque on the wheels 59. Deceleration is only possible withstandard friction-type brakes connected to the wheel axle.

In view of the above, both the output-coupled and input-coupled PSDtransmissions have certain limitations and inefficiencies, such thatadditional developments and improvements would be desirable to furtherexpand the technical and commercial viability of PSD transmissions.

BRIEF SUMMARY OF THE INVENTION

The present invention provides power split drive (PSD) transmissionsthat are suitable for use in automotive applications, and exhibitimproved operating efficiencies as a result of having an energy recoverycapability.

A PSD transmission of this invention includes a mechanical transmissionsystem for mechanically transmitting mechanical power between arotatable input shaft (for example, a shaft connected to an engine) anda rotatable output shaft (for example, a shaft connected to a wheelaxle), and a hydraulic transmission system containing a fluid forhydraulically transmitting hydraulic power between the input shaft andthe output shaft, and at least a third shaft operatively interconnectedto one of the mechanical and hydraulic transmission systems. Thehydraulic transmission system is operatively coupled by at least a firstplanetary gear train to the mechanical transmission system. According tothe invention, the PSD further comprises means operatively associatedwith at least one of the mechanical and hydraulic transmission systemsfor storing and releasing energy within the PSD transmission, the energystoring and releasing means comprising a flywheel or an accumulator or acombination thereof.

According to one aspect of the invention, the energy storing meanscomprises one or more flywheels and/or accumulators. In the case of theformer, the flywheel may be coupled to the input shaft and/or the thirdshaft. In the case of the latter, at least one accumulator is preferablyfluidically coupled to a positive displacement device operable to storeenergy from the power split transmission by operating as a pump to storea portion of the fluid at an elevated pressure in the accumulator andoperable to release energy to the power split transmission by operatingas a motor driven by the fluid released from the accumulator. PSDtransmissions of this invention are preferably capable of combining thevariability of a CVT and the efficiency of mechanical transmissionsystems, along with an energy storage capability that provides potentialbenefits for both on road and off road vehicles.

According to additional aspects of the invention, the PSD transmissioncan by categorized based on whether the hydraulic transmission system(defining the hydraulic path of the PSD transmission) is coupled to theinput (input coupled) or to the output (output coupled) of themechanical transmission system, and the characteristics of the planetarygear train (basic, dual-stage, or compound). Accordingly, the PSDtransmission may be a basic output-coupled embodiment in which case thehydraulic transmission system is mechanically coupled through the firstplanetary gear train to the input shaft and mechanically coupled to theoutput shaft, or a compound output-coupled embodiment in which case thehydraulic transmission system is mechanically coupled through the firstplanetary gear train to the input shaft and mechanically coupled througha second planetary gear train to the output shaft, or a basicinput-coupled embodiment in which case the hydraulic transmission systemis mechanically coupled to the input shaft and mechanically coupledthrough the first planetary gear train to the output shaft, or amultistage input-coupled embodiment in which case the hydraulictransmission system is mechanically coupled to the input shaft and thefirst planetary gear train is a multistage planetary gear train thatmechanically couples the hydraulic transmission system to the outputshaft.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are charts showing the basic power modes and transmissionfamilies of PSD transmissions.

FIG. 3 schematically represents a basic output-coupled PSD transmissionknown in the prior art.

FIG. 4 schematically represents a basic output-coupled PSD transmissionequipped with a flywheel and accumulator that provide an energy recoverycapability in accordance with a first embodiment of this invention.

FIG. 5 schematically represents a compound output-coupled PSDtransmission equipped with a flywheel and accumulator that provide anenergy recovery capability in accordance with a second embodiment ofthis invention.

FIG. 6 schematically represents a basic output-coupled PSD transmissionequipped with only an accumulator to provide an energy recoverycapability in accordance with a third embodiment of this invention.

FIG. 7 schematically represents a compound output-coupled PSDtransmission equipped with only an accumulator to provide an energyrecovery capability in accordance with a fourth embodiment of thisinvention.

FIG. 8 schematically represents a basic input-coupled PSD transmissionknown in the prior art.

FIG. 9 schematically represents a basic input-coupled PSD transmissionequipped with a flywheel and accumulator that provide an energy recoverycapability in accordance with a fifth embodiment of this invention.

FIG. 10 schematically represents a multistage input-coupled PSDtransmission equipped with a flywheel and accumulator that provide anenergy recovery capability in accordance with a sixth embodiment of thisinvention.

FIG. 11 schematically represents a basic input-coupled PSD transmissionequipped with only an accumulator to provide an energy recoverycapability in accordance with a seventh embodiment of this invention.

FIG. 12 schematically represents a multistage input-coupled PSDtransmission equipped with only an accumulator to provide an energyrecovery capability in accordance with an eighth embodiment of thisinvention.

FIGS. 13A through 13D identify four power flow modes of output-coupledPSD transmissions of this invention.

FIGS. 14A through 14G identify eight power flow modes of input-coupledPSD transmissions of this invention.

FIG. 15 schematically represents Standard Control and Secondary Controlschemes for use with the input-coupled PSD transmissions of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 4 through 7 and 9 through 12 schematically represent PSDtransmissions (PST's) comprising mechanical and hydrostatic transmissionsystems, in which the hydrostatic transmission systems (the hydraulicpath of the PSD transmission) are coupled to either the output(output-coupled) or input (input-coupled) of its mechanical transmissionsystem. In each case, the PSD transmission is capable of capturing andreleasing energy, preferably over the entire speed range of the PSDtransmission, through the use of a storage recovery and release systemthat comprises one or more accumulators integrated into the hydrostatictransmission system and/or one or more flywheels integrated into themechanical transmission system. Because the invention takes the form ofmultiple embodiments that employ functionally similar components,consistent reference numbers are used where noted to identifyfunctionally similar components.

FIGS. 4 through 7 represent output-coupled PSD transmissions with energyrecovery capabilities according to four embodiments of the invention.FIG. 4 represents a basic output-coupled PSD transmission 110 of thisinvention, with energy storage capability provided in the form of twoaccumulators 120 and 122 and an optional flywheel 124. The flywheel 124is mechanically coupled to a combustion engine 111 (or other suitablepower source) through a freewheel clutch 126 and is mounted on anengine-driven shaft 128 to a planetary (epicyclic) gear train 114 thatforms part of the mechanical transmission system of the PSD transmission110. As with conventional planetary gear trains, the planetary geartrain 114 is represented as comprising a sun gear 130, a ring gear 132circumscribing the sun gear 130, and planet gears 134 carried on aplanet gear carrier 136 and simultaneously in mesh with the sun gear 130and ring gear 132. In FIG. 4, the planet gear carrier 136 is showncoupled to the engine-driven shaft 128 and the sun gear 130 is mountedon an output shaft 138 of the mechanical transmission system coupled tothe drive axle and wheels 119 of the vehicle.

The accumulators 120 and 122 are high pressure (HP) and low pressure(LP) accumulators, respectively, integrated into a hydrostatictransmission system 112 of the PSD transmission 110. The hydrostatictransmission system 112 further includes first and second positivedisplacement units 116 and 118 (Units 1 and 2), respectively, eachcoupled to fluid lines A and B. The first and second units 116 and 118are coupled to the ring gear 132 and output shaft 138, respectively,with shafts 140 and 142 and suitable gearing (having gear ratios of i₁and i₂, respectively). The high pressure and low pressure accumulators120 and 122 are directly connected to the fluid lines A and line B,respectively. Other than pressure relief valves (not shown), additionalvalves could be used but are not required for the embodiment of FIG. 4as shown.

FIG. 5 shows a compound PSD transmission 210 that is essentially thesame as the basic PSD transmission 110 of FIG. 4 (hence, the usage ofthe same reference numbers for its components), but with the furtheraddition of a second planetary gear train 115 through which thehydrostatic transmission system 112 is coupled to the output shaft 138,instead of the shaft 142 and gearing of FIG. 4. FIGS. 6 and 7 furthershow basic and compound variations 310 and 410, respectively, of the PSDtransmission 110 of FIG. 4 (hence, the usage of the same referencenumbers for their components), but with energy storage means 120/122/124coupled to the engine-driven shaft 128 by a shaft 144 and suitablegearing. While the energy storage means 120/122/124 is shown in FIGS. 6and 7 as a single accumulator, one or more flywheels (with suitableclutches) could be used, as could combinations of accumulator(s) andflywheel(s).

With the inclusion of the flywheel 124 coupled to the engine-drivenshaft 128 and/or the accumulators 120 and 122 within the hydrostatictransmission system 112, brake energy can be stored and used for vehiclepropulsion. There are three main types of power flow modes associatedwith the output-coupled PSD transmissions of FIGS. 4 through 7:propulsion via the engine 111 or flywheel 124; propulsion via theaccumulator(s) 120/122; and energy storage using the flywheel 124 and/oraccumulator(s) 120/122. Energy recovery can occur in both the additiveand recirculation operating modes of the PSD transmissions 110, 210,310, and 410.

In the power additive mode, propulsion of the vehicle can beaccomplished through the use of power from the engine 111 or theflywheel 124. The power from the engine/flywheel 111/124 is delivered bythe shaft 128 to the planet gear carrier 136 of the planetary gear train114. Within the gear train 114, the power is split between a shaft 146(coupled to the ring gear 132) and the output shaft 138 (coupled to thesun gear 130). The power in the output shaft 138 is transferred to thewheels 119 through a mechanical path (e.g., the shaft 138 and wheelaxle), while the power in the shaft 146 is transferred to thehydrostatic transmission system 112 (via the associated gear set).Within the hydrostatic transmission system 112, the displacement units116 and 118 operate as a pump and motor, respectively, when thetransmission 110 is in the power additive mode.

In the power additive mode, propulsion of the vehicle can also beaccomplished through the use of stored energy from the accumulators 120and 122. The stored energy is in the form of pressurized fluid withinthe HP accumulator 120 and is transferred to the unit 118, which acts asa motor to drive the shaft 142. The capability of driving thedisplacement unit 118 with pressurized fluid to propel the vehiclerequires a change in the control of the displacements of the units 116and 118 beyond that necessary for the prior art output-coupled PSDtransmission 10 of FIG. 3.

Energy storage can also occur in the power additive mode using theflywheel 124 and/or accumulator(s) 120/122. When braking is desired,power is transferred to the hydrostatic transmission system 112 throughthe gearing connecting the shaft 142 to the output shaft 138, andtransferred through the output shaft 138 to the planetary gear train114. When storing energy with the accumulators 120 and 122, thedisplacement unit 118 operates as a pump and the displacement unit 116operates at zero displacement. When storing energy with the flywheel124, the unit 118 operates as a pump and the unit 116 operates as amotor, transferring power to the flywheel 124 via the shaft 146 andplanetary gear train 114. The freewheel clutch 126 allows the flywheel124 to accelerate and rotate at speeds faster than the engine 111, thusincreasing the energy storage capability of the flywheel 124.

In the power recirculation mode, propulsion of the vehicle can beaccomplished through the use of power from the engine 111 or theflywheel 124. The power from the engine/flywheel 111/124 is summed inthe planetary gear train 114 through the shafts 128 and 138. The powerdelivered by the shaft 138 is split at gearing that couples the shaft138 to the shaft 142 of the displacement unit 118, with part of thepower being transferred to the wheels 119 to propel the vehicle and theremainder recirculated through the hydrostatic transmission system 112to allow for further increase of vehicle speed. Within the hydrostatictransmission system 112, the displacement unit 118 operates as a motordriven by the fluid output of the displacement unit 116.

In the power recirculation mode, propulsion of the vehicle can also beaccomplished through the use of stored energy from the HP accumulator120, whose pressurized fluid is transferred to the displacement unit 118acting as a motor.

Finally, the power recirculation mode also allows for energy storageusing the flywheel 124 and/or accumulators 120/122. When braking isdesired, power is transferred to the hydrostatic transmission system 112through the gearing connecting the shafts 138 and 142, and transferredthrough the shaft 138 to the planetary gear train 114. When storingenergy with the HP accumulator 120, the displacement unit 118 operatesas a pump and the displacement unit 116 operates at either zerodisplacement or as a pump. When storing energy with the flywheel 124,the displacement unit 116 operates as a pump and the mode of thedisplacement unit 118 determines if power is recirculated to the shaft138 or stored in the accumulator 120. Again, the freewheel clutch 126allows the flywheel 124 to accelerate and rotate at speeds faster thanthe engine 111.

In view of the above, in contrast to the conventional output-coupledtransmission 10 shown in FIG. 3, energy recovery with the PSDtransmissions 110, 210, 310, and 410 of FIGS. 4 through 7 involves theuse of different control of the displacement units 116 and 118. Thedisplacement unit 116 is used to control the pressure in line A based ona lowest desired minimum pressure corresponding to the minimum fluidvolume in the high pressure accumulator 120, and the displacement unit118 is used to control the vehicle speed based on the desired vehiclespeed. Therefore, as in the conventional output-coupled transmission ofFIG. 3, a feedback signal from the vehicle speed is necessary forcontrolling the displacement unit 118. Additionally, a pressure signalfrom line A is employed to control the displacement unit 116.

As discussed above and represented in further detail in FIGS. 13Athrough 13D, the displacement unit 116 either operates as a pump or isfreewheeling, while the displacement unit 118 either operates as a motoror pump. As such, the displacement unit 118 is an over-center unit thatallows its operation as a motor or pump, while the displacement unit 116may but does not require an over-center operation capability. Theimplementation of the described control concept, referred to as thesecondary control principle, allows the implementation of the high andlow pressure accumulators 120 and 122 for use in energy recovery withoutthe need for switching valves in fluid lines A and B. However, the useof switching valves could be used and is therefore also within the scopeof the invention.

The basic output-coupled PSD transmission 110 of FIG. 4 is designed insuch a way that in forward speeds the transmission 110 runs solely inpower additive mode, without ever entering full mechanical mode or powerrecirculation mode. As full mechanical mode is approached the speed ofthe displacement unit 116 decreases to zero. In the secondary controlledsystem of the output-coupled PSD transmission 110, this will result inthe emptying of the accumulator 120 as flow exits to the displacementunit 118 but no flow is produced by the unit 116. In reverse thetransmission 110 operates in power recirculation mode, with the unit 118operating over center as a motor. Due to the direct coupling of the unit118 to the shaft 138 and wheels 119, energy capture is possible in allspeed ranges.

The operational modes are shown in more detail in FIGS. 13A through 13Dand discussed below in particular reference to the basic output-coupledPSD transmission 110 of FIG. 4, though it should be understood that theoperating principles also apply to the transmissions 210, 310, and 410of FIGS. 5 through 7. During propulsion (Power Flow Modes 0, I, and IIin FIGS. 13A, 13B and 13C, respectively) and braking (Power Flow ModeIII in FIG. 13D), the fluid line A is always the high pressure line andfluid line B is always the low pressure line. At zero vehicle speed,v_(veh), the displacement of the unit 118 is zero. As the desiredvehicle speed (v_(veh,des)) is increased, the displacement of unit 118increases, delivering a torque to the wheels 119. In propulsion, thevehicle may be powered by either the engine 111 or the high pressureaccumulator 122.

In propulsion, if the pressure in line A is at the minimum operatingpressure (p_(1HP)), the transmission 110 will be running as a PSDtransmission powered by the engine 111 (Power Flow Mode I in FIG. 13B).Unit 116 is in pumping mode and unit 118 is in motoring mode. Unit 116holds the pressure in the fluid line A at the minimum operatingpressure, p_(1HP). As the pressure in line A drops below p_(1HP), thedisplacement of unit 116 increases, filling the accumulator 120 andincreasing the pressure in line A. As v_(veh,des) increases, unit 118increases in displacement, producing a torque on the wheels 119 of thevehicle. Power is transferred from the engine-driven shaft 128 bothmechanically (PA) and hydraulically (PC). If, in propulsion, thepressure in line A is higher than p_(1HP), the transmission 110 ispowered solely by the high pressure accumulator 120 (Power Flow Mode IIin FIG. 13C). The unit 116 is freewheeling and the unit 118 is inmotoring mode. The unit 116 is controlled by the pressure in line A tozero displacement, and the unit 118 is controlled by v_(veh,des). Nopower is transferred from the engine 111.

During braking (Power Flow Mode III in FIG. 13D), the unit 118 operatesover center, producing a negative torque at the wheels 119 and acting inpumping mode. As the pressure in line A increases above p_(1HP), unit116 is controlled to zero displacement, and flow from unit 118 is usedto charge the high pressure accumulator 120 to store brake energy. Thedisplacement of unit 118, and therefore the amount of brake torque, iscontrolled by v_(veh,des). If the high pressure accumulator 120 isfilled, fluid flow from the unit 118 is directed to the low pressureaccumulator 122 through a pressure relief valve. Therefore, the size ofthe accumulators 120 and 122 play an important role in brake energyrecovery. No power is transferred back to unit 116 or the shaft 128. Theuse of additional braking may be necessary for both safety and highbraking demands.

Reverse is accomplished by controlling the displacement unit 118 overcenter. If the pressure in fluid line A is at p_(1HP), the transmission110 runs in power recirculation mode (Power Flow Mode 0 in FIG. 13A). Ifthe pressure in line A is above p_(1HP), the vehicle will be powered bythe high pressure accumulator 120, as in Power Flow Mode II (FIG. 13C).

FIGS. 9 through 12 represent input-coupled PSD transmissions with energyrecovery capabilities according to four additional embodiments of theinvention. FIG. 9 represents a basic input-coupled PSD transmission 150of this invention, with the energy storage capability provided in theform of two accumulators 160 and 162 and an optional flywheel 164. Theflywheel 164 is mechanically coupled to a combustion engine 151 (orother suitable power source) through a freewheel clutch 166 that allowsthe engine 151 to drop to speeds lower than the rotational speed of theshaft 168 on which the flywheel 164 is mounted. The shaft 168 is coupledto a planetary (epicyclic) gear train 154 that forms part of themechanical transmission system of the PSD transmission 150. As withconventional planetary gear trains, the planetary gear train 154 isrepresented as comprising a sun gear 170, a ring gear 172 circumscribingthe sun gear 170, and planet gears 174 carried on a planet gear carrier176 and simultaneously in mesh with the sun gear 170 and ring gear 172.In contrast to the output-coupled PSD transmissions 110, 210, 310, and410 of FIGS. 4 through 7, the input-coupled PSD transmission 150 of FIG.9 has its sun gear 170 coupled to the shaft 168 and its planet gearcarrier 176 mounted on a shaft 178 coupled to the drive axle and wheels159 of the vehicle.

The accumulators 160 and 162 are high pressure (HP) and low pressure(LP) accumulators, respectively, integrated into a hydrostatictransmission system 152 of the PSD transmission 150. The high pressureaccumulator 160 is for energy storage and the low pressure accumulator162 is for low pressure fluid storage. The hydrostatic transmissionsystem 152 further includes first and second positive displacement units156 and 158 (Units 1 and 2), respectively, each coupled to fluid lines Aand B. The first and second units 156 and 158 are coupled to the ringgear 172 and shaft 168, respectively, with shafts 180 and 182 andsuitable gearing. The high pressure and low pressure accumulators 162and 164 are connected to the fluid lines A and line B through a valveblock 163 whose components are schematically detailed in FIG. 15. Otherthan pressure relief valves (not shown), additional valves may be usedbut are not required for the embodiment of FIG. 9 as shown.

FIG. 10 shows a multistage input-coupled PSD transmission 250 that isessentially the same as the basic input-coupled PSD transmission 150 ofFIG. 9 (hence, the usage of the same reference numbers for itscomponents), except that the planetary gear train 154 is a dual-stageplanetary gear train capable of smaller or larger gear ratios, as knownin the art. FIGS. 11 and 12 further show basic and multistage variations350 and 450, respectively, of the PSD transmission 150 of FIG. 9 (hence,the usage of the same reference numbers for their components), but withenergy storage means 160/162/164 coupled to the engine-driven shaft 168by a shaft 184 and suitable gearing. While the energy storage means160/162/164 is represented in FIGS. 11 and 12 as a single accumulator,one or more flywheels (with clutches) could be used, as couldcombinations of accumulator(s) and flywheel(s).

With the inclusion of the flywheel 164 coupled to the engine-drivenshaft 168 and/or the accumulators 160 and 162 within the hydrostatictransmission system 152, brake energy can be stored and used for vehiclepropulsion. Three main types of power flow modes are associated with theinput-coupled PSD transmissions of FIGS. 9 through 12: propulsion viathe engine 151 or flywheel 164; propulsion via the accumulator(s)160/162; and braking using the flywheel 164 and/or the accumulator(s)160/162. It is also possible to combine some forms of propulsion, forexample, propulsion can also occur via a combination of the engine 151and accumulator(s) 160/162, or a combination of the flywheel 164 andaccumulators 160/162. Energy recovery can occur in both the additive andrecirculation operating modes of the PSD transmissions 150, 250, 350,and 450.

In the power recirculation mode, propulsion of the vehicle can beaccomplished through the use of power from the engine 151 or theflywheel 164. The power from the engine/flywheel 151/164 is carriedthrough the shaft 168 and split in the planetary gear train 154 to theshaft 178 (coupled to the planet gear carrier 176) and a shaft 186coupled to the hydrostatic transmission system 152 through the ring gear172. The power at the shaft 178 is transferred to the wheels 159 topropel the vehicle, while the remainder is recirculated through thehydrostatic transmission system 152. Within the hydrostatic transmissionsystem 152, the displacement unit 158 operates as a pump and thedisplacement unit 156 operates as a motor.

The power recirculation mode also allows for energy storage using theflywheel 164. When braking is desired, power is transferred from theshaft 178 to the engine shaft 168, which drives both the flywheel 164and the displacement unit 156. The unit 156 operates as a pump while theunit 158 operates as a motor, transferring power to the shaft 186. Powerfrom the shafts 178 and 186 is summed in the planetary gear train 154and recirculated to the shaft 168. The freewheel clutch 166 allows theflywheel 164 to accelerate and rotate at speeds faster than the engine151.

In the power additive mode, propulsion of the vehicle can beaccomplished through the use of power from the engine 151 or theflywheel 164. Power from the engine/flywheel 151/164 is summed with thepower of the hydrostatic transmission system 152 in the planetary geartrain 154 through the shaft 168 (coupled to the sun gear 130) and shaft186 (coupled to the ring gear 132). Within the hydrostatic transmissionsystem 152, the displacement unit 156 operates as a pump and thedisplacement unit 158 operates as a motor.

Propulsion of the vehicle can also be accomplished in the power additivemode through the use of stored energy from the HP accumulator 160. Thestored energy is transferred to the units 156 and 158, both of whichoperate as motors to drive, respectively, the shaft 180 coupled to theshaft 168 and the shaft 182 coupled to the ring gear 172. If theflywheel 164 also has stored energy, power from the flywheel 164 and HPaccumulator 160 can be used to propel the vehicle. Such a capabilityentails a change to the control of the displacements of the units 156and 158 compared to FIG. 8.

Finally, energy storage can also occur in the power additive mode usingthe flywheel 164 and/or accumulator(s) 160/162. When braking is desired,power from the wheels 159 is transferred to the displacement unit 158through the shaft 186 and to the flywheel 164 and displacement unit 156through the shaft 168. When storing energy with the HP accumulator 160,both displacement units 156 and 158 operate as pumps to deliver fluid tothe accumulator 160. The freewheel clutch 166 allows the flywheel 164 toaccelerate and rotate at speeds faster than the engine 151.

A “Standard Control” scheme of the valve block 163 is represented on thelefthand side of FIG. 15. The valve block 163 includes a check valve V1that prevents the HP accumulator 160 from discharging when in position0, and a second valve V2 that prevents the HP accumulator 160 from beingcharged when in position 0. A third valve V3 prevents high pressure flowfrom entering the LP accumulator 162 when in position 0, and a checkvalve V4 prevents high pressure flow from entering the LP accumulator162. FIG. 15 does not show the low pressure system of the hydrostatictransmission system 152 or high pressure relief valves that arepreferably employed to direct flow to the LP accumulator 162 if the HPaccumulator 160 is filled. Various options could be considered forhandling the low pressure fluid of the hydrostatic transmission system152.

A “Secondary Control” scheme is also represented in FIG. 15. With thisscheme, the displacements of both displacement units 156 and 158 arecontrolled simultaneously by separate control signals. With theSecondary Control scheme, the differential system pressure is not simplya reactionary function of the load torque on the wheels 159, but isregulated to always operate at or above an allowable minimum pressurelevel, p_(1HP), chosen so that the displacement unit 158 can produce aminimum allowable amount of torque at the wheel axle. The high pressureline is always fluid line A during Secondary Control. Measurement ofvehicle speed and pressure in line A are used as feedback signals.

The full mechanical point, v_(mech), marks the transition from StandardControl to Secondary Control. At vehicle speeds (v_(veh)) belowv_(mech), the input-coupled PSD transmissions 150, 250, 350, and 450 arecontrolled with the Standard Control scheme. v_(mech) occurs atapproximately 33% of the maximum vehicle velocity, and is described inCarl et al. (noted previously, and whose contents are incorporatedherein by reference) as the vehicle speed at which the displacement unit158 experiences zero rotational velocity:v _(mech)≡(n _(A)/(1−i _(o))·i _(axle))·r _(tire)where n_(A) is the rotational speed of the shaft 168, i_(o) is thestanding gear ratio of the planetary gear train 154, i_(axle) is thegear ratio of the wheel axle, and r_(tire) is the dynamic rolling radiusof the wheels 159. The vehicle velocity feedback signal, v_(veh), andshaft speed, n_(A), are all that is required to indicate that v_(mech)has been reached and the transition from Standard Control to SecondaryControl can occur. For any speed above v_(mech), the transmissions 150,250, 350, and 450 can be controlled with either Standard Control orSecondary Control, though energy recovery occurs only when the SecondaryControl scheme is active. When the feedback signal v_(veh) is greaterthan or equal to the calculated value v_(mech) as defined in theequation above, the control scheme can be switched from Standard toSecondary Control. If the control scheme switch occurs, valves V1through V3 are all energized.

The Power Flow Modes of the input-coupled PSD transmissions 150, 250,350, and 450 are summarized in FIGS. 14A through 14G. It should be notedthat the HP and LP accumulators 160 and 162 are coupled to the fluidlines A and B through the valve block 163, and not directly to the fluidlines A and B as shown in the simplified schematics used in FIGS. 14Athrough 14G. Each Power Flow Mode has a unique set of control signals,one signal for each unit.

During Modes 0 through III, under Standard Control the displacements ofthe displacement units 156 and 158 are controlled sequentially toachieve a desired vehicle speed, v_(veh,des). During Mode IV underSecondary Control, the vehicle is propelled with engine power, thedisplacement of the unit 156 is controlled to maintain p_(1HP) in the HPaccumulator 160, and the displacement of the unit 158 is simultaneouslycontrolled to achieve the desired vehicle speed, v_(veh,des). The torqueoutput of the unit 158 can be adjusted by changing the displacement ofthe unit 158, such that the resulting aiding torque on the wheel axlecan be increased as necessary to attain the desired vehicle speed.During Mode V of the Secondary Control, the vehicle decelerates, thedisplacement of the unit 156 is controlled to regulate the speed of theshaft 168 to some desired speed, n_(A,des), by exerting an appropriateamount of resistive torque on the shaft 168, and the displacement of theunit 158 is simultaneously controlled to achieve a desired vehiclespeed, v_(veh,des), by exerting an appropriate amount of resistivetorque on the axle shaft through the shaft 186. As a result, thepressure level of the HP accumulator 160 increases as the vehicle'skinetic energy is stored in the form of pressurized fluid within theaccumulator 160.

During Mode VI of Secondary Control, the vehicle is propelled withhydraulic power. The displacement of the unit 156 is again controlled toregulate the speed of the shaft 168 to some desired speed, n_(A,des), byexerting an appropriate amount of aiding torque on the shaft 168. Theengine speed is allowed to drop below n_(A,des), while simultaneouslythe displacement of the unit 158 is again controlled to achieve adesired vehicle speed, v_(veh,des), by exerting an appropriate amount ofaiding torque on the axle shaft through the shaft 186. As a result, thepressure level within the HP accumulator 160 decreases as theaccumulator's potential energy is transferred to vehicle kinetic energy.

At the full mechanical point, v_(mech), the input-coupled PSDtransmissions 150, 250, 350, and 450 switch naturally from powerrecirculation mode to power additive mode. Power from the wheels 159travels through the planetary gear train 154 during braking maneuvers.During braking in power recirculation mode, the unit 158 operates as amotor and does not transfer energy to the HP accumulator 160. Similarly,although the unit 156 is operating as a pump during this time, it doesnot transfer energy to the HP accumulator 156. Consider that powerenters the planetary gear train 154 through the shaft 178 duringrecirculation mode braking. If the engine 151 is incapable of storingenergy due to the freewheel clutch 166, all power transferred to theunit 156 from the shaft 168 must be transferred to the shaft 186 throughthe unit 158 in order to balance the power flow within the planetarygear train 154, that is, power into the planetary gear train 154 must beequal to the power out of the planetary gear train 154. Storing energyin the HP accumulator 160 requires that not all power from the shaft 168is transferred back into the shaft 186, creating a power unbalancewithin the planetary gear train 154. Therefore energy storage duringpower recirculation mode braking is not possible. During power additivemode, the units 156 and 158 operate as pumps during braking maneuversand are capable of transferring energy to the HP accumulator 160. Powerand therefore energy transferred into the planetary gear train 154through shaft 178 is transferred to the HP accumulator 160 through theunits 156 and 158 via the shafts 168 and 186, respectively, such thatthe power balance within the planetary gear train 154 is satisfied.Since energy capture is only possible during power additive mode, it isadvantageous to decrease the engine speed allowing for a lower value ofn_(A,des) during braking maneuvers to extend the energy capture regionas much as possible, since lowering n_(A) lowers v_(mech).

The potential for fuel savings through the use of the output-coupled andinput-coupled PSD transmissions of FIGS. 4 and 9 were modeled using asoftware library entitled Power Split Drive Design (PSDD), built in aMatlab/Simulink environment and reported in Mikeska et al., “VirtualPrototyping of Power Split Drives,” Proc. Bath Workshop on PowerTransmission and Motion Control PTMC (2002), Bath, UK, p. 95-111, whosecontents are incorporated herein by reference. The model simulated theuse of a reference vehicle with a 225 kW engine and a maximum drivingspeed of 160 km/h under an urban dynamometer driving cycle. At theconclusion of the simulation, the output-coupled PSD transmission 110 ofFIG. 4 was indicated as being the most advantageous solution in terms ofenergy savings for the reference vehicle and the studied drive cycle. Ascompared to a conventional input-coupled PSD transmission (FIG. 8), theoutput-coupled and input-coupled PSD transmissions 110 and 150 of thisinvention were predicted to have energy efficiency savings of about35.27% and about 16.19%. The output-coupled PSD transmission 110 furthershowed a distinct advantage over the input-coupled PSD transmission 150in its ability to capture energy at all speed ranges during thesimulated drive cycle. This result may be attributable to the directconnection between the displacement unit 118 and the wheel axle thatexists in the output-coupled system, eliminating the need for power flowto exist in the planetary gear train 114 during energy capturemaneuvers. The input-coupled system was limited to energy capture onlyat vehicle speeds greater than v_(mech), roughly 33% of the maximumvelocity.

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the physical configuration of the PSD transmissionscould differ from those shown, and components capable of functionsimilar to the components described could be use. Therefore, the scopeof the invention is to be limited only by the following claims.

1. A power split transmission capable of energy recovery, the powersplit transmission comprising: a mechanical transmission systemcomprising a rotatable input shaft and at least a first planetary geartrain; the first planetary gear train comprising a sun gear coupled tothe input shaft, a carrier directly coupled to a rotatable output shaft[178], and a ring gear directly coupled to a rotatable third shaft[186], the first planetary gear train being adapted for mechanicallytransmitting mechanical power between the third shaft, the input shaftand the output shaft [178]; a hydraulic transmission system [152]containing a fluid adapted for hydraulically transmitting hydraulicpower to and from the input shaft and the third shaft [186], thehydraulic transmission system [152] being continuously operativelycoupled by the third shaft to the first planetary gear train; and atleast one of the mechanical and hydraulic transmission systems [154,152]; [154, 152] comprising means for storing and releasing energywithin the power split transmission, the energy storing and releasingmeans comprising a flywheel or an accumulator or a combination thereof;wherein the mechanical transmission system [154,158] and the hydraulictransmission system [152] are coupled to the input and output shafts[168, 178] so that the power split transmission is operable torecirculate power that is selectively received by the hydraulictransmission system [152] from the output shaft [178] through the firstplanetary gear train and the third shaft [186], and then delivered bythe hydraulic transmission system [152] through the input shaft to thefirst planetary gear train.
 2. The power split transmission according toclaim 1, further comprising means for operating the mechanical andhydraulic transmission systems so that the power split transmission hasmultiple operating modes, a first operating mode characterized by theoutput shaft receiving power from the input shaft through at least oneof the mechanical and hydraulic transmission systems, a second operatingmode characterized by the output shaft receiving power from the energystoring and releasing means, a third operating mode characterized by theoutput shaft receiving power from the input shaft through at least oneof the mechanical and hydraulic transmission systems and from the energystoring and releasing means thereof, and a fourth operating modecharacterized by the output shaft delivering power to the energy storingand releasing means.
 3. The power split transmission according to claim1, further comprising means for operating the mechanical and hydraulictransmission systems so that the power split transmission has multipleoperating modes, a first operating mode characterized by the outputshaft receiving mechanical power through the mechanical transmissionsystem and hydraulic power through the hydraulic transmission system, asecond operating mode characterized by the output shaft receiving onlymechanical power through the mechanical transmission system, and a thirdoperating mode characterized by the mechanical transmission systemreceiving hydraulic power through the hydraulic transmission system, thehydraulic transmission system receiving mechanical power through themechanical transmission system, and the output shaft receiving less thanall of the mechanical power of the mechanical transmission system andless than all of the hydraulic power from the hydraulic transmissionsystem.
 4. The power split transmission according to claim 3, whereinthe energy storing and releasing means uses a portion of the mechanicalpower to generate stored energy when the power split transmission isoperating in the third operating mode.
 5. The power split transmissionaccording to claim 1, wherein the energy storing means comprises aflywheel coupled to at least one of the input and third shafts.
 6. Thepower split transmission according to claim 1, wherein the energystoring means consists of a flywheel coupled to the input shaft.
 7. Thepower split transmission according to claim 1, wherein the energystoring means consists of a flywheel coupled to the third shaft.
 8. Thepower split transmission according to claim 1, wherein the energystoring means comprises at least one accumulator.
 9. The power splittransmission according to claim 8, the hydraulic transmission systemfurther comprising a positive displacement device fluidically coupled tothe accumulator, the positive displacement device being operable tostore energy from the power split transmission by operating as a pump tostore a portion of the fluid at an elevated pressure in the accumulatorand operable to release energy to the power split transmission byoperating as a motor driven by the fluid released from the accumulator.10. The power split transmission according to claim 9, wherein thehydraulic transmission system is mechanically coupled through the firstplanetary gear train to the input shaft and mechanically coupled to theoutput shaft.
 11. The power split transmission according to claim 9,wherein the hydraulic transmission system is mechanically coupled to theinput shaft and mechanically coupled through the first planetary geartrain to the output shaft.
 12. The power split transmission according toclaim 9, wherein the hydraulic transmission system is mechanicallycoupled to the input shaft and the first planetary gear train is amulti-stage planetary gear train that mechanically couples the hydraulictransmission system to the output shaft.
 13. The power splittransmission according to claim 8, the hydraulic transmission systemfurther comprising a positive displacement device fluidically coupled tothe accumulator and mechanically coupled to the mechanical transmissionsystem.
 14. The power split transmission according to claim 8, whereinthe energy storing means does not comprise a flywheel.
 15. The powersplit transmission according to claim 8, wherein the hydraulictransmission system is mechanically coupled through the first planetarygear train to the input shaft and mechanically coupled to the outputshaft.
 16. The power split transmission according to claim 8, whereinthe hydraulic transmission system is mechanically coupled to the inputshaft and mechanically coupled through the first planetary gear train tothe output shaft.
 17. The power split transmission according to claim 8,wherein the hydraulic transmission system is mechanically coupled to theinput shaft and the first planetary gear train is a multi-stageplanetary gear train that mechanically couples the hydraulictransmission system to the output shaft.
 18. The power splittransmission according to claim 1, wherein the power split transmissiondoes not comprise a clutch adapted to decouple the hydraulictransmission system and the mechanical transmission system from eachother.